The present invention relates to an image signal decoding system and, more particularly, to an image signal decoding system capable of decoding modified Huffman codes at a high speed.
Heretofore, run-length codes have been employed as image-compressing codes for use with high-speed facsimile machines. As a run-length coding scheme, the Comite Consultatif Internationale Telegraphique et Telegraph (CCITT) recommends a modified Huffman coding scheme. The modified Huffman coding scheme is a coding scheme for code-compressing image signals such as a code that has encoded an arbitrary run-length number by combining 54 kinds of make-up codes which encodes up to 1728 pixels by every 64 pixels with 128 kinds of terminating codes in which every 64 pixels from pixel 0 to pixel 63 by one pixel, so as to allow the run-length number to correspond white run-length number and black run-length number to white pixels and black pixels, respectively.
Such a system for decoding image signals encoded by the modified Huffman coding scheme of this type is known, for example, by an image signal decoding system as disclosed in Japanese Patent Publication (Kokoku) No. 16,071/1989. The image signal decoding system as disclosed in this prior patent publication is arranged to perform the decoding of white or black run-length information of facsimile data encoded by the modified Huffman coding scheme by a small fixed memory capacity. In this system, the kind and run length of codes are prepared so as to correspond to each of the modified Huffman codes (hereinafter referred to as "MH codes"), and the MH codes are decoded by providing data of 10 bits obtained by processing the received MH codes as a ROM (read only memory) access address, reading the data in the ROM and developing it into dots.
However, the image signal decoding system in the prior art technology presents the problem that no attention is paid to the processing speed for decoding, so that the demand for processing image signals at a high speed is not met.